Felting is the process by which wool fiber is matted into a fabric. Wool fibers have scales and are also crimped. When these fibers are wetted and softened and then rubbed together they grab each other and are permanently bound together. This happens when you put a wool sweater in a washer and agitate it -- it comes out much smaller and also denser, because the fibers have bound together.
Felting is done by a chemical process in industry. It is also sometimes done with barbed needles, which grab individual fibers and drag them against their neighbors, thereby binding them.
Historically, the felting process used mercury nitrate. Over time, exposure to mercury could lead to dementia. Hatters, who used large amounts of felt, were often exposed to mercury vapours over long periods of time. Hence the expression, "Mad as a hatter".
Felting is an ancient form of wool processing, the earliest examples dating to Neolithic times. It is still practiced by nomadic peoples in Asia, where rugs, tents and clothing are regularly made.
- See also: felt, Mad Hatter.